FB is notoriously glitchy. Doesn't sound like your nephew has done anything to keep you from posting.

Fb can decided at random whether to do something or not, it has no logic. A few weeks ago it randomly showed a picture of the Dali Llama, that defiantly wasn't her profile picture, weird and highly amusing.

The problem with FB is that it's really easy to be sensitive and take offense at something done by a FB friend - even if done innocently.

This could be a glitch, or you could've been grouped in the "family" category that has restricted access, or he could've accidentally changed a setting to limit someone else and it limits you, etc. And, as you said, your nephew may want to keep some things private from his family. You can't take it personally. You haven't been unfriended or blocked so he's still friends with you. And please don't say anything. That just puts him on the spot to explain something that he shouldn't have to explain.